What is the method of shotcrete? The shotcrete method involves spraying concrete or mortar onto a surface using high pressure. It is typically used in situations where conventional formwork is difficult, impractical, or where structural reinforcement is needed. There are two primary shotcrete methods: dry-mix and wet-mix , and they each follow distinct processes.

　　I. Dry-Mix Shotcrete Method :

　　In this method, the dry components of the concrete (cement, sand, aggregate) are mixed and placed into the shotcrete machine, but water is not added until the point of application. Here's the step-by-step process:

　　Steps :

　　1. Dry mix loading : Dry materials (cement, sand, and possibly aggregate) are placed into a hopper of the shotcrete machine.

　　2. Air propulsion : Compressed air is used to blow the dry material through a hose toward the nozzle.

　　3. Water addition : At the nozzle, water is introduced and mixed with the dry material just before being sprayed onto the surface.

　　4. Application : The concrete is sprayed at high velocity onto the surface, where it consolidates and adheres due to the force of application.

　　Advantages of Dry-Mix :

　　- Ideal for remote locations where water access might be limited.

　　- The operator has more control over the water-cement ratio, which can be adjusted at the nozzle.

　　- Less material waste during start-up and shutdown.

　　Disadvantages :

　　- It can create more dust during the process.

　　- Requires skilled operators to ensure the proper water mix at the nozzle.

　　II. Wet-Mix Shotcrete Method :

　　In the wet-mix method, all ingredients (including water) are mixed before being placed into the shotcrete machine. The pre-mixed concrete is then pumped through a hose and sprayed onto the surface with compressed air.

　　Steps :

　　1. Wet mix preparation : The concrete mix (cement, aggregate, sand, water, and any admixtures) is prepared in a concrete mixer.

　　2. Pumping : The wet concrete is pumped into a hose via a shotcrete pump.

　　3. Compressed air : At the nozzle, compressed air is introduced to propel the concrete out of the hose at high velocity.

　　4. Application : The material is sprayed onto the surface, forming a dense, compact layer.

　　Advantages of Wet-Mix :

　　- Less rebound (material that bounces off the surface during application), leading to less waste.

　　- Suitable for larger-volume applications because of the continuous nature of pumping.

　　- Better workability and control over the concrete mix.

　　Disadvantages :

　　- Wet-mix requires more equipment for mixing, pumping, and applying the concrete.

　　- Less flexibility in adjusting the water-cement ratio during the process.

　　Common Steps in Both Methods :

　　- Surface preparation : The surface is cleaned, roughened, and dampened to ensure good adhesion of the shotcrete. Sometimes reinforcing materials like wire mesh are applied to support the shotcrete.

　　- Application : The operator sprays the shotcrete in layers, ensuring that each layer adheres to the surface properly.

　　- Finishing : The surface of the shotcrete can be smoothed or shaped, depending on the application. For structural uses, it is often left rough for further finishing.

　　- Curing : Proper curing (keeping the shotcrete moist) is necessary to ensure the concrete develops its full strength.

　　Both methods are versatile and can be used in construction, repair, and reinforcement of a wide range of structures, including tunnels, retaining walls, swimming pools, and slope stabilization projects.